Chemiosmosis is the source of the ATP produced in photophosphorylation

In Chapter 7 we considered the chemiosmotic mechanism for ATP formation in the mitochondrion. The chemiosmotic mechanism also operates in photophosphorylation (Figure 8.11). In chloroplasts, as in mitochondria, electron transport through the redox chain is coupled to the transport of pro-

fyw, 8.11 Chloroplasts Form ATP Chemiosmotically

|V "J) Protons (H+) pumped across the thylakoid membrane \ j from the stroma during electron transport make the ' ' interior of the thylakoid more acidic than the stroma. Driven by this pH difference, the protons diffuse back to the stroma through ATP synthase channels, which couple the energy of proton diffusion to the formation of ATP from ADP + Pi.

Thylakoid interior

High concentration of H+ (low pH)

Electron transport chain

ATP synthesis

Thylakoid interior

High concentration of H+ (low pH)

Electron transport chain

ATP synthesis

Protons are actively transported to interior of thylakoid compartment by electrons from photosystem II.

Chemiosmosis

Stroma

Low concentration of H+ (high pH)

Protons are actively transported to interior of thylakoid compartment by electrons from photosystem II.

Stroma

Low concentration of H+ (high pH)

tons (H+) across the thylakoid membrane, which results in a proton gradient across the membrane.

The electron carriers in the thylakoid membranes are oriented so that protons move from the stroma into the interior of the thylakoid. The interior compartment becomes acidic with respect to the stroma. When there is sufficient light, the ratio of H+ inside versus outside a thylakoid is usually 10,000:1, which is a difference of 4 pH units. This difference leads to the diffusion of H+ back out of the thylakoid interior through specific protein channels in the thylakoid membrane. These channels are enzymes—ATP synthases—that couple the diffusion of protons to the formation of ATP, just as in mitochondria (see Figure 7.12). In the chloroplast, the ATP is generated in the stroma, where it will be available to provide the energy for the fixation of CO2 in the production of carbohydrate by the Calvin-Benson cycle.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • YOHANNES
    How many hydrogen pump chemiosmosis in mitochondria?
    5 years ago
  • Elsie
    How many hydrogen produced in chemiosmosis?
    5 years ago
  • Joona
    Why atp produced in stroma?
    5 years ago
  • BARBARA
    How is atp made photophosphoraltion?
    5 years ago
  • ada
    Who demonstrated the chemiosmotic mechanism also operates in chloroplasts?
    5 years ago
  • poppy
    Where is H pumped across the thylakoid membrane?
    5 years ago
  • romeo
    What are produced when protons are pumped across the membranes of mitochondreia?
    4 years ago
  • Gilly
    Is ATP generated by a chemosmotic membrane?
    4 years ago
  • stacey
    How are atps made in chemiosmotic photophosphorylation?
    3 years ago

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